As a solid-state image pickup device of this kind, there has hitherto been known a CCD image sensor having pixels arranged in a matrix fashion.
FIG. 10 is a plan view generally showing an example of an arrangement of a conventional CCD image sensor.
This CCD image sensor includes a semiconductor substrate (Si substrate, semiconductor chip) 10 on which there is provided an image pickup region 20. In this image pickup region, there are disposed photosensors (photodiodes) 22 as photoelectric conversion portions, each of which serves as a pixel, a plurality of vertical transfer registers 24 and channel stop regions 26 at every photosensor column. Further, a horizontal transfer register 32 and an output portion 34 are provided at the outside of the image pickup region 20.
The outside of the image pickup region 20 is used as a peripheral region 21 in which bus lines and the like are placed.
Signal electric charges generated by each photosensor 22 are read to the vertical transfer register 24, transferred to the vertical direction at every photosensor column and thereby outputted to the horizontal transfer register 32 in turn.
The horizontal transfer register 32 transfers signal electric charges, transferred from the respective photosensors 22 by the vertical transfer register 24, to the horizontal direction at every row and outputs the transferred signal electric charges to the output portion 34 in turn.
The output portion 34 converts the signal electric charges, transferred by the horizontal transfer register 32, into voltage signals in turn and outputs the voltage signals after it has processed the voltage signals in such a suitable way as to amplify them.
The channel stop region 26 is adapted to block leakage of signals between the adjacent photosensor columns.
FIG. 11 is a cross-sectional view showing structures of elements within the CCD image sensor shown in FIG. 10 and shows the cross-section taken along the line a-a in FIG. 10.
As illustrated, on the upper layer of the semiconductor substrate (Si substrate) 10, there are formed the photosensor 22, the vertical transfer register 24 and the channel stop region 26. A transfer electrode (polysilicon film) 44 of the vertical transfer register 24 is deposited on the upper surface of the semiconductor substrate 10 through an insulating film (silicon oxide film) 42, and a light-shielding film 46 is attached to the upper layer of this transfer electrode.
This light-shielding film 46 has an opening portion 46A formed at its portion corresponding to the light-receiving region of the photosensor 22, and light can be introduced into the photosensor 22 through this opening portion 46A.
Also, the photosensor 22 includes a P+ layer 22A of an upper layer and an N layer of a lower layer. Holes generated by photoelectric conversion are supplied to the P+ layer 22 and signal electric charges are generated from the N layer 22B.
The signal electric charges generated by the N layer 22B are accumulated in a depletion layer formed on the lower layer of the N layer 22B and they are read from the photosensor 22 to the side of the vertical transfer register 24 as a read gate portion provided between the photosensor 22 and the vertical transfer register 24 is operated.
Further, in the inside region of the semiconductor substrate 10, there is provided an overflow barrier (OFB) by which signal electric charges generated by each photosensor 22 are stored in the lower portion region of the N layer 22B.
This overflow barrier 28 forms a potential barrier in the inside region of the semiconductor substrate 10 by adjusting a distribution of impurities within the semiconductor substrate to block leakage of signal electric charges. Also, when light of an excess light amount becomes incident on this solid-state image pickup device, signal electric charges excessively generated by the photosensor 22 are discharged to the rear side of the semiconductor substrate 10 through this overflow barrier 28.
In the above-mentioned CCD solid-state image pickup device, as the unit pixel is miniaturized increasingly, development of technology for increasing sensitivity per unit area becomes the most urgent need.
As one of the means of such technology, it is considered that the overflow barrier is not formed at the position with a depth of approximately 3 μm from the surface of the Si substrate like the prior art but the overflow barrier should be formed at the deeper position (for example, at the position with a depth of 5 μm to 10 μm from the surface of the substrate).
In this state, when potential of the conventional vertical transfer register is formed, its potential distributions are obtained as shown in FIGS. 12 and 13.
More specifically, FIG. 12 is an explanatory diagram showing potential distributions of the photosensor and the vertical transfer register along the cross-section of the substrate in which the vertical axis represents the depth of potential and the horizontal axis represents the depth from the surface of the substrate. Then, a solid line characteristic curve A shows a potential distribution of the photosensor portion and a broken line characteristic curve B shows a potential distribution of the vertical transfer register portion.
Also, FIG. 13 is an explanatory diagrams showing the potential distribution in the photosensor region in a three-dimensional fashion in which the X axis represents the horizontal direction, the Y axis represents the depth direction of potential and the Z axis represents the depth direction of the substrate, respectively. A plane formed by the X axis and the Y axis represents the surface of the substrate.
The vertical axis in FIG. 12 and the Y axis of FIG. 13 mean the fact that potential becomes higher in the lower direction. Also, numerical values on graduation on each axis are those adjusted for convenience.
In such potential distributions, the potential position of the photosensor and the potential position of the lower layer portion of the vertical transfer register become equal to each other in the deep portion of the substrate.
Accordingly, in such state, electric charges photoelectrically-converted by the sensor region are diffused in the lateral direction (shown by an arrow D in FIG. 13). As a result, a problem called crosstalk arises, in which electric charges are caused to enter the sensor region of the adjacent pixel.
It is an object of the present invention to provide a solid-state image pickup device in which crosstalk between the adjacent pixels can be prevented effectively even when an overflow barrier is provided at the deep portion of a substrate.